1. Field of the Invention
The present invention relates to a method for preparing poly(2,5-benzimidazole), and more particularly to a method for preparing poly(2,5-benzimidazole) having proton conductivity, a poly(2,5-benzimidazole) electrolyte membrane prepared according to the method, and a fuel cell comprising the poly(2,5-benzimidazole) electrolyte membrane.
2. Description of the Related Art
Fuel cells as electrochemical cells convert energy that is generated by an oxidation reaction of fuel to electrical energy. On an anode of a fuel cell, organic fuel such as methanol, formaldehyde, or formic acid is oxidized to carbon dioxide, and on a cathode, air or oxygen is reduced to water. A high specific energy of organic fuel renders the fuel cell more attractive. For example, the specific energy of methanol is 6,232 wh/kg.
Fuel cells comprise a membrane electrode assembly (MEA) including an anode layer and a cathode layer, and a polymer electrolyte membrane (PEM) that is interposed between the two electrode layers and plays a role as a proton transfer medium. As the conductive polymer electrolyte of a fuel cell, a fluorine-containing polymer electrolyte membrane such as a perfluoro carbon sulfonic acid membrane (Nafion™ manufactured by the Dupont Company) has chemical stability, high ionic conductivity and good mechanical properties, and is generally used.
However, a fluorine-containing polymer electrolyte has a disadvantage in that it is prepared by a complicated process, and has a high manufacturing cost. Further, since a fluorine-containing polymer electrolyte has a heat-resistance limit of less than 100° C., a cooling system for reforming gas and a removing system for carbon monoxide are needed when it is used as a power source for an automobile, a commercial small-sized power plant, and a portable power plant. The proton conductivity deteriorates, and infiltration of methanol occurs at a high temperature of more than 80° C. or under low relative humidity of less than 60%.
Therefore, a sulfonated polymer such as sulfonated polyimide, polystyrene, polyphenylene, or polyetheretherketone (PEEK) has been researched as a polymer electrolyte membrane as a replacement for a fluorine-containing polymer electrolyte membrane. The Celanese company has developed poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole] (polybenzimidazole, PBI) which can overcome shortcomings of the fluorine-containing polymer such as Nafion™. PBI has relatively lower methanol permeability, and can be manufactured at a lower cost than the fluorine-containing polymer. Further, it has conductivity at a high temperature of more than 100° C., and therefore it can be used as an electrolyte membrane material for a fuel cell at a high temperature, and carbon monoxide (CO) poisoning can be reduced.
Poly(2,5-benzimidazole) has a similar structure and conductivity to PBI, but it can be a polymer with a high molecular weight so that a polymer electrolyte having more improved mechanical properties for a fuel cell can be prepared. Poly(2,5-benzimidazole) is synthesized by heating 3,4-diaminobenzoic acid using polyphosphoric acid or a mixture of P2O5 and phosphoric acid as a dehydrating reagent. The aforementioned dehydrating reagent has a high viscosity and is hard to handle, and a polymerization is carried out at a high temperature of over 200° C. Further, a resultant polymer synthesized using the dehydrating reagent forms a rigid lump and is hard to purify. Also, the phosphoric acid of the conventional dehydrating reagent cannot be removed easily when it is doped on the polymer or is present in a polymer chain. Therefore, in order to use poly(2,5-benzimidazole) as a polymer electrolyte membrane for a fuel cell, a new synthesis method of the polymer must be researched.